Overload relays are electrical switches typically employed in industrial settings to protect electrical equipment from damage due to overheating in turn caused by excessive current flow. In a typical case, the electrical equipment is a three-phase motor which is connected to a power source through another relay commonly referred to as a contactor. The contactor is controlled by another switch which is typically remotely located.
In a conventional setup, an overload relay is connected in series with the control switch for, in turn, control of the contactor. When an overload condition is detected by the overload relay, the overload relay operates the switch which, in turn, de-energizes the contactor which cuts the source of power to the electrical equipment, thereby preventing damage to the equipment.
In the past, overload relays have utilized resistive heaters for each phase which are in heat transfer relation with a bimetal element, which in turn controls a switch. When an overload is sensed, as, for example, when there is sufficient heat input from the resistive heater to the bimetal element, the bimetal element opens its associated switch to de-energize the contractor coil and disconnect the associated piece of electrical equipment from the source of power.
More recently, the resistive heater-bimetal element type of relay has been supplanted by electrical overload relays. See, for example, commonly assigned U.S. Pat. No. 5,179,495 issued Jan. 12, 1993, to Zuzuly, the entire disclosure of which is herein incorporated by reference. Outputs of such circuitry typically are relatively low power and, as a consequence, in order for the output to control the-contactor coil current, an electrical-mechanical switch may be required.
In one case, an overload relay, once tripped, will remain in an open position, preventing the flow of current to the contactor, and must be manually reset. Usually, a push button is employed so that an operator may push the push button to cause a reset of the system, closing the contacts of the overload relay to again allow current to flow to the contactor coil which in turn will close the contactor contacts and provide current to the electrical equipment.
During periods of overload, prior art overload relays typically trip after the overload has occurred for a period of time. This delayed trip helps prevent nuisance tripping during minor fluctuations or noise in the signals. However, these relays typically do not issue a warning or alarm during the interim period between initial detection of an overload and the tripping of the relay. Consequently, the user is unaware that overloads are occurring and cannot take corrective action to prevent tripping.
During periods of phase loss, prior art relays typically trip after there has been a phase loss for a relatively short period of time. However, prior art relays use RC timing circuits to determine these delay intervals. Since RC timing circuits are susceptible to heat and humidity changes, the RC time constant can vary with these changes, which can lead to nuisance or false relay trips, which, in turn, can damage the motor.